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Full text of "Handbook Of Chemical Engineering - I"

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CRYSTALLIZATION                                   407
The mixed precipitate of double salt and ferrous ferricyanide is filtered out, washed to remove sulphates and sulphocyanides and decomposed by boiling with sodium carbonate, forming the sodium ferrocyanide and setting free the ammonia which is distilled off and collected. The sodium ferrocyanide solution is filtered off from the iron-lime precipitate, concentrated and crystallized as already described on page 404. Unfortunately the double salt precipitate is very difficult to wash, with the result that frequently considerable amounts of sulphates pass into the finished liquor as sodium sulphate. The first crop of crystals is normally a pure, high-grade sodium ferrocyanide. The first mother liquor is concentrated to proper density and crystallized in like manner.
If the washing of the double salt has been at all thorough the crystals from the first mother liquor are of good grade but if not they are added to first or fine liquor and recrystallized as first-grade product. The second mother liquor is in turn concentrated and crystallized, the crystals going back into the next batch of first mother liquor and the third mother liquor being again concentrated and crystallized. These crystals go into second mother liquor and the fourth mother liquor which usually contains not more than 3 to 5 per cent ferrocyanide is either discarded or worked up for its sulphocyanide content, by this time often as high as 25 to 30 per cent. All first-crop crystals are dried in centrifugal machines and washed with steam and are ready for packing and shipment. In plants where strings are used for crystallization, the strings are pulled out where possible, the large bodies of crystals rinsed free from mother liquor, drained and air dried. Crystals to be returned to process are simply stripped and drained.
Methods 1, 2, and 3 (p. 399) cover the most common and most important means of crystallization. Method 4 (simple cooling) by itself is a special case, more applicable commercially to organic compounds than to inorganic. A typical example of its application is in the separation of isomeric compounds having different melting points, e.g., separation of meta dichlorbenzol from paradichlor benzol. The meta compound boils within 3 of the para and is completely soluble in the para compound, the meta being, however, a solid at ordinary temperatures while the para is a liquid. By cooling the solution of meta in para to 0C. the meta nearly all separates in beautiful crystals and the para can be drained off nearly pure. This method has a fairly wide application in similar cases and will solve many troublesome problems of separation.
Method 5 (addition of a solvent for the impurities) is of technical interest though as yet of only limited commercial application. It has been used principally in the preparation of some of the rarer sugars, many of which are very difficult to crystallize from the solution in which they are prepared exactly as is the case in recovering sucrose from molasses. It has been found that by addition of glacial acetic acid the impurities are made more soluble while the sugars crystallize out readily. Cases are known where concentrated solutions have stood for months without developing crystals, which after mixing with glacial acetic acid, produced a crop of crystals of unusual purity over night. Like many other chemical processes, the principle of the operation has been known for many years, but its practical application has only recently been made, and even now has not been greatly extended.
Method 6 is another rather special case, which, however, finds extensive commercial application in certain lines. At first sight it may be thought to be no more than an application of precipitation but precipitation is generally brought about by a reaction between the precipitating agent and substance in solution to form a new compound. Rather Method 4 is intended to apply to cases such as are represented by separation of compounds by immiscible solvents, the compound, or the impurity as the case may be, being soluble in both solvents, while the body to be separated is soluble in only one. Method 6, however, applies to many cases where the separated body comes out in